The present invention relates to an audio system, and more particularly to an audio system which suppresses standing waves produced in a room to provide an improved sound effect as perceived.
A conventionally known audio device of this type is disclosed in Japanese Patent Laid-Open Publication No. Hei 9(1997)-22293.
This audio device allows audio signals to pass through adaptive filters to supply the signals to reproducing loudspeakers. Then, sound outputted from the reproducing loudspeakers is measured by means of a microphone arranged at a listening location. Frequency characteristics of the adaptive filters are appropriately adjusted so that the difference between the measured signal thus obtained and said audio signal becomes zero, whereby standing waves uncomfortable as perceived are prevented from being produced.
Standing waves uncomfortable to a listener are characterized by the resonance frequency of a transfer function of the room. Accordingly, the audio signal is filtered in advance by an adaptive filter which is able to cancel out the effects of the transfer function and the audio signal thus filtered is supplied to the reproducing loudspeaker, whereby uncomfortable standing waves are prevented from being produced in the room.
However, in the aforementioned conventional audio device, the audio signal is not supplied directly to the reproducing loudspeaker, but is filtered by means of the aforementioned adaptive filter and then supplied to the reproducing loudspeaker.
Accordingly, in some cases, the filtering process produced wave distortion in the audio signal, or such frequency components exceeding the reproduction capability of the reproducing loudspeaker were mixed in the audio signal. Consequently, there was a problem that the reproducing loudspeaker produced distorted sound or unnatural sound as perceived.
The present invention has been developed in view of the aforementioned problem and an object of the present invention is to provide an audio system which enables creating of a natural sound field space as perceived and suppressing of standing waves.
A first aspect of the present invention is to provide an audio system comprising a signal source for outputting audio signals, a first sound source for receiving the audio signals supplied by the signal source to reproduce and output sound, compensation means for generating compensation signals for suppressing standing waves by signal-processing the audio signals, and a second sound source for receiving the compensation signals supplied by the compensation means to reproduce and output sound for suppressing standing waves, wherein the compensation means comprises correlator means for determining a cross-correlation function between a transfer function from the first sound source to a listening location and a transfer function from the second sound source to the listening location, filter means having frequency characteristics based on the cross-correlation function generated by the correlator means, and signal inverting means, the filter means filters the audio signals and the signal inverting means inverts signals generated through the filtering, whereby compensation signals to be supplied to the second sound source are generated.
According to the above-mentioned constructions, the standing wave resulted from the transfer function from the first sound source to the listening location is canceled out by the sound which the second sound source outputs upon receiving the compensation signal. Consequently, sound outputted by the first sound source, that is, the sound reproduced based on the intrinsic audio signal reaches the listening location. Accordingly, a sound field space which is not affected by the standing wave uncomfortable as perceived is created at the listening location.
Furthermore, the cross-correlation function represents the similarity between the transfer function from the first sound source to the listening location and the transfer function from the second sound source to the listening location. Therefore, setting the filter means to the frequency characteristics which are characterized by this cross-correlation function causes the filter means to generate a signal having frequency characteristics close to those of the standing wave. Furthermore, inverting the signal by the signal inverting means generates a signal which causes the second sound source to generate sound having an opposite phase with respect to the standing wave, that is, a compensation signal.
A second aspect of the present invention is to provide an audio system comprising a signal source for outputting audio signals, a first sound source for receiving the audio signals supplied by the signal source to reproduce and output sound, compensation means for generating compensation signals for suppressing standing waves by signal-processing the audio signals, and a second sound source for receiving the compensation signals supplied by the compensation means to reproduce and output sound for suppressing standing waves, the audio system further comprising convolution operational means for performing a convolution operation of a transfer function from the second sound source to the listening location and a transfer function of a predetermined filter means, correlator means for determining a cross-correlation function between an operational result of the convolution operational method, and a transfer function from the first sound source to the listening location, extracting means for extracting feature information regarding phases and gain characteristics of the cross-correlation function for the transfer function of the predetermined filter means, filter means to be set to frequency characteristics characterized by the feature information extracted by the extracting means, and signal inverting means, wherein the filter means is used for filtering the audio signals and the signal inverting means inverts signals generated through the filtering, whereby compensation signals to be supplied to the second sound source are generated.
The cross-correlation function obtained through the operation of the convolution operational means and the correlator means represents the similarity between the first transfer function from the first sound source to the listening location and the second transfer function from the second sound source to the listening location. Therefore, setting the filter means to the frequency characteristics which are characterized by this cross-correlation function causes the filter means to generate a signal having frequency characteristics close to those of the standing wave. Furthermore, inverting the signal by the signal inverting means generates a signal which causes the second sound source to generate sound having an opposite phase with respect to the standing wave, that is, a compensation signal.